1. Field of the Invention
This invention relates to a developing assembly used in an electrophotographic apparatus, an electrostatic recording apparatus, a magnetic recording apparatus or the like, and a process cartridge and an image-forming method which make use of the developing assembly.
More particularly, this invention relates to a developing assembly used in an image-forming apparatus such as a copying machine, a printer, a facsimile machine or a plotter, in which a toner image (developer image) is previously formed on an image-bearing member and thereafter the toner image is transferred to a recording medium such as a transfer material to form an image; a process cartridge having such a developing assembly and detachably mountable to such an image-forming apparatus; and an image-forming method making use of the developing assembly.
2. Related Background Art
In recent years, in image-forming methods carried out by electrophotography, contact charging assemblies have been proposed in large number and have been put into practical use as assemblies used to charge charging objects, such as latent-image-bearing members electrostatically, because of their advantages of lower ozone generation and lower power consumption than corona charging assemblies.
The contact charging assembly is an assembly in which a conductive charging member (contact charging member or contact charging assembly) of a roller type (charging roller), a fur brush type, a magnetic-brush type or a blade type is brought into contact with a charging object member, such as an image-bearing member, and a stated bias voltage is applied to this contact charging member to charge the surface of the charging object member electrostatically to the stated polarity and potential.
The charging roller is formed using a conductive or medium-resistance rubber material or foam. In some rollers, such a rubber material or foam is provided in layers to attain the desired characteristics.
The charging roller is provided with an elasticity in order to ensure the state of uniform contact between it and the charging object member. For this reason, it has a great frictional resistance, and in many cases it is driven to follow-up with, or at some difference in speed from, the rotation of the charging object member. Hence, any attempt to perform direct-injection charging may inevitably cause a decrease in absolute chargeability, a contact unevenness due to a shortage in contact performance and roller shape and a charging unevenness due to any deposits on the charging object member.
FIG. 2 is a graph showing examples of charging efficiency of contact charging in electrophotography. The bias voltage applied to the contact charging member is plotted along the abscissa, and the charge potential of the charging object (hereinafter “photosensitive member”), obtained there, is plotted along the ordinate.
Charge characteristics in the case of roller charging are represented by line A. That is, the surface potential of the photosensitive member begins to rise after the applied voltage exceeds a threshold value of about −500 V, and, at voltages higher than such threshold value, the photosensitive member surface potential increases linearly at a slope of 1 with respect to the applied voltage. This threshold value voltage is defined as the charging start voltage Vth. Accordingly, when the photosensitive member is charged to −500 V, it is common to employ a method in which a DC voltage of −1,000 V is applied, or, in addition to the charging voltage of −500 V, an AC voltage of, e.g., a peak-to-peak voltage of 1,200 V is applied so as to provide a potential difference larger than the discharge threshold value, to converge the photosensitive member potential to the charge potential.
Namely, in order to obtain a photosensitive member surface potential Vd that is required in electrophotography, a DC voltage of “Vd+Vth”, what is higher than is necessary, must be applied to the charging roller. The charging performed by applying only a DC voltage to the contact charging member in this way is called “DC charging”.
In DC charging, however, it has been difficult to control the potential of the photosensitive member at the desired value because the resistance value of the contact charging member varies depending on environmental variations and also because the Vth varies with changes in layer thickness caused by the abrasion of the photosensitive member.
When the AC charging is performed in order to achieve uniform charging, ozone may be generated, the electric field generated by AC voltage may cause a vibrating noise (AC charging sound) between the contact charging member and the photosensitive member, and any discharging may remarkably cause deterioration or the like of the surface of the photosensitive member. Therefore, this practice has come into question.
The fur brush charging method is one in which, using as a contact charging member a member having a conductive-fiber brush portion (a fur brush charging assembly), the conductive-fiber brush portion is brought into contact with a photosensitive member as the charging object, and a stated charging bias is applied to the conductive-fiber brush portion to charge the surface of the photosensitive member electrostatically to the stated polarity and potential.
For the fur brush charging assembly, a fixed type and a roll type have been put into practical use. The one in which medium-resistance fibers formed in a folded pile on a base cloth have been bonded to an electrode is the fixed type. The roll type is formed by winding pile around a mandrel. Those having a fiber density of about 100 fibers/mm2 are obtained relatively with ease, but are still insufficient for contact performance in order to perform well uniform charging by direct-injection charging. In order to well perform uniform charging by direct-injection charging, the fur brush charging assembly must be made to have a velocity different from that of the photosensitive member; the difference is so large as to make machine construction difficult. This is not realistic.
Charge characteristics of this fur brush charging at the time of application of DC voltage are as shown by line B in FIG. 2. Hence, in the case of fur brush charging, too, in both the fixed type and the roll type, the charging is performed under the application of a high charging bias voltage in many cases to utilize the phenomenon of discharging.
In contrast to these, the magnetic-brush charging method is one in which, using as a contact charging member a member having a magnetic-brush portion (a magnetic-brush charging assembly) formed by confining conductive magnetic particles magnetically by means of a magnet roll, the magnetic-brush portion is brought into contact with a photosensitive member as the charging object, and a stated charging bias is applied to charge the surface of the photosensitive member electrostatically to the stated polarity and potential. In the case of this magnetic-brush charging, its charging mechanism is predominantly governed by a direct-injection charging mechanism.
As the conductive magnetic particles with which the magnetic-brush portion is constituted, those having a particle diameter of from 5 μm to 50 μm may be used, and a sufficient velocity differential from that of the photosensitive member may be provided, whereby almost uniform direct-injection charging can be performed.
Charge characteristics of the magnetic-brush charging at the time of the application of DC voltage are shown by line C in FIG. 2. As shown in FIG. 2, it is possible to attain a charge potential substantially proportional to the applied bias voltage.
Magnetic-brush charging, however, may also cause a difficulty that the conductive magnetic particles constituting the magnetic-brush portion come off to adhere to the photosensitive member. Thus, it is sought to provide an assembly for simple, stable and uniform charging, which can be operated by the direct-injection charging mechanism causing substantially no discharge products, such as ozone, and producing uniform charging at a low applied voltage.
Meanwhile, from the viewpoint of resource saving and waste reduction and in the sense of effective utilization of toners (developers), an image-forming method which does not bring any waste toner is desired. For example, what is called toner reuse has been put into practical use, in which, after a latent image on a latent-image-bearing member is developed with a toner to form a toner image as a visible image and the toner image is transferred to a recording medium such as paper, any toner having remained on the latent-image-bearing member without being transferred to the recording medium is removed by cleaning by various methods, and this toner is circulated into a developing assembly and reused. There, however, has been a problem that pressing a cleaning member against the latent-image-bearing member surface causes the latent-image-bearing member to wear to make the latent-image-bearing member have a short lifetime. Also, when viewed from the standpoint of the apparatus, the image-forming apparatus must be made larger in size in order to provide such a toner reuse assembly and a cleaning assembly. This has been a bottleneck in attempts to make the apparatus compact.
As a countermeasure therefor, as a system which does not bring any waste toner, also proposed is a technique called a cleaning-at-development or cleanerless system. Conventional techniques concerning the cleaning-at-development or cleanerless system are, as disclosed in Japanese Patent Application Laid-open No. 5-2287, focused on positive memory or negative memory appearing on images because of the influence of the transfer residual toner on images. However, in these days where electrophotography is greatly utilized, it has become necessary to transfer toner images to various recording media. In this sense, such techniques have not been satisfactory for various recording media.
The related art discloses techniques concerning the cleanerless system as seen in Japanese Patent Applications Laid-open Nos. 2-302772, 5-2289, 5-53482 and 5-61383. These, however, neither mention any desirable image-forming methods, nor refer to how the toner is constituted.
As developing systems in which the cleaning-at-development or cleanerless system is preferably applied, basically having no cleaning assembly, it has ever been considered essential for the system to be so made up that the latent-image-bearing member surface is rubbed with the toner and the toner-carrying member. Accordingly, studies have largely been made on contact developing systems in which the toner or developer comes into contact with a latent-image-bearing member. This is because, in order to collect the transfer residual toner in a developing means, it is considered advantageous for the system to be so made up that the toner or developer comes into contact with and rubs the latent-image-bearing member. However, in the cleaning-at-development or cleanerless process making use of a contact development system, its long-term service tends to cause deterioration of toner, deterioration of the toner-carrying member surface and deterioration or wear of the latent-image-bearing member surface, but any satisfactory solution has not been made for running performance. Accordingly, it has been sought to provide a cleaning-at-development system according to a non-contact developing system.
In this cleaning-at-development system or cleanerless image-forming method, the point is that the charge polarity and charge quantity of the transfer residual toner on the photosensitive member is controlled so that the transfer residual toner can stably be collected in the step of development and the collected toner may not make the developing performance poor. Accordingly, the charge polarity and charge quantity of the transfer residual toner on the photosensitive member is controlled by means of the charging member. This will be described specifically in the case of a commonly available laser beam printer.
In the case of reverse development making use of a charging member for applying a voltage with negative polarity, a negatively chargeable photosensitive member, and a negatively chargeable toner, in the transfer step thereof, the image rendered visible is transferred to the recording medium by means of a transfer member to which a voltage with positive polarity is applied. The charge polarity of the transfer residual toner varies because of its relation to the type of the recording medium (differences in thickness, resistance, dielectric constant and so forth) and the areas of images to produce a toner having positive charges and also even a toner having negative charges. However, when the photosensitive member is charged with a charging member having a negative polarity, the charge polarity of the transfer residual toner can uniformly be adjusted to the negative polarity together with the photosensitive member surface even if the polarity of the transfer residual toner has been shifted to the positive polarity in the transfer step. Hence, when reversal development is employed as the developing system, the transfer residual toner, which stands negatively charged, remains at light-area potential areas to be developed by toner. On the other hand, the toner present at dark-area potential areas not to be developed by toner is attracted toward the toner carrying member in relation to the development electric field and is collected without remaining on the photosensitive member having a dark-area potential. That is, the cleaning-at-development or cleanerless image-forming method can be established by controlling the charge polarity of transfer residual toner simultaneously with the charging of the photosensitive member by means of the charging member.
However, where the transfer residual toner has adhered to or mingled with the contact charging member beyond the contact charging member's capacity to control toner's charge polarity, it becomes impossible to uniformly adjust the charge polarity of the transfer residual toner, making it difficult to collect the toner in the step of development. Also, even where the transfer residual toner has been collected on the toner-carrying member by mechanical force such as rubbing, the transfer residual toner may adversely affect the triboelectric chargeability of toner on the toner-carrying member, resulting in a lowering of developing performance, unless the charge of the transfer residual toner has not uniformly been adjusted. More specifically, in the cleaning-at-development or cleanerless image-forming method, the charge control performance at the time the transfer residual toner passes the charging member and the manner in which the transfer residual toner adheres to or mingles with the charging member are closely concerned with the running performance and image quality characteristics.
In the cleaning-at-development image-forming method, cleaning-at-development performance can be improved by improving charge control performance required when the transfer residual toner passes the charging member. As a proposal therefor, Japanese Patent Application Laid-open No. 11-15206 discloses an image-forming method making use of a toner having toner particles containing a specific carbon black and a specific azo type iron compound and having inorganic fine powder. It is also proposed, in the cleaning-at-development image-forming method, to improve cleaning-at-development performance by reducing the quantity of transfer residual toner, using a toner having a superior transfer efficiency, the shape factors of which have been specified. However, the contact charging used here also applies the discharge charging mechanism, which is not the direct injection charging mechanism, and has the above problem ascribable to discharge charging. Moreover, these proposals may be effective for keeping the charging performance of the contact charging member from decreasing because of the transfer residual toner, but can not be expected to be effective for actively improving the charging performance.
In addition, among commercially available electrophotographic printers, cleaning-at-development image-forming apparatus are also available in which a roller member, coming into contact with the photosensitive member, is provided between the transfer step and the charging step so that the performance of collecting the transfer residual toner at development can be assisted or controlled. Such image-forming apparatus have good cleaning-at-development performance and the waste toner can sharply be reduced, but involve a high cost and may adversely affect the advantage inherent in the cleaning-at-development system also in view of compact construction.
In order to prevent uneven charging to effect stable and uniform charging, the contact charging member may be coated with a powder on its surface coming into contact with the surface of the member to be charged. Such constitution is disclosed in Japanese Patent Publication No. 7-99442. However, the contact charging member (charging roller) is so constructed as to be follow-up rotated as the charging object member (photosensitive member) is rotated (without no velocity differential drive), and hence may remarkably cause less ozone products compared with corona charging assemblies such as Scorotron. However, the principle of charging is still chiefly the discharge charging mechanism like the case of the roller charging described previously. In particular, a voltage formed by superimposing AC voltage on DC voltage is applied in order to attain more stable charging uniformity, and hence the ozone products caused by discharging may more greatly occur. Accordingly, when the apparatus is used over a long period of time, difficulties, such as smeared images due to ozone products, tend to come out. Moreover, when the above construction is applied in cleanerless image-forming apparatus, any inclusion of the transfer residual toner makes it difficult for the coated powder, to stand adhered uniformly to the charging member, so that the effect of carrying out uniform charging may decrease.
Japanese Patent Application Laid-open No. 5-150539 also discloses that, in an image-forming method making use of contact charging, at least image-developing particles and conductive fine particles having an average particle diameter smaller than that of the image-developing particles are contained in a toner in order to prevent any charging obstruction which may be caused when toner particles or silica particles having not completely been removed by blade cleaning come to adhere to and accumulate on the surface of the charging means during repetition of image formation for a long time. However, the contact charging used here, or proximity charging, applies the discharge charging mechanism, which is not the direct injection charging mechanism, and has the above problem ascribable to discharge charging. Moreover, when this construction is applied in the cleanerless image-forming apparatus, the influence on charging performance of a larger quantity of conductive fine particles and transfer residual toner in the charging step than the apparatus having a cleaning mechanism, the influence on the collection of these large-quantity conductive fine particles and transfer residual toner in the developing step, and the influence on the developer's developing performance that is exercised by the conductive fine particles and transfer residual toner thus collected, is not addressed. Furthermore, when the direct injection charging mechanism is applied in the contact charging, the conductive fine particles can not be fed to the contact charging member in the necessary quantity, thereby causing faulty charging due to the influence of the transfer residual toner.
In proximity charging, it is also difficult to uniformly charge the photosensitive member because of the large-quantity conductive fine particles and transfer residual toner, and the effect of leveling patterns of the transfer residual toner cannot be obtained, to cause a pattern ghost because the transfer residual toner may shut out pattern-imagewise exposure light. In-machine contamination due to developer may further occur when a power source is instantaneously turned off or a paper jam occurs during image formation.
As countermeasures for these, Japanese Patent Application Laid-open No. 10-307456 discloses an image-forming apparatus in which a developer containing toner particles and conductive charge-accelerating particles having a particle diameter, which is ½ or smaller than the particle diameter of toner, is applied in a cleaning-at-development, image-forming method, making use of the direct injection charging mechanism. According to this proposal, a cleaning-at-development, image-forming apparatus can be obtained which can sharply reduce the quantity of waste toner and is advantageous for making the apparatus compact at a low cost, and good images are obtainable without causing any faulty charging and any shut-out or scattering of imagewise exposure light. It, however, is sought to make a further improvement.
Japanese Patent Application Laid-open No. 10-307421 also discloses an image-forming apparatus in which a developer containing conductive particles having a particle diameter which is 1/50 to ½ of the particle diameter of the toner is applied in a cleaning-at-development image-forming method, making use of the direct injection charging mechanism and the conductive particles are made to have a transfer accelerating effect.
Japanese Patent Application Laid-open No. 10-307455 still also discloses that, a conductive fine powder is controlled to have particle diameter not larger than the size of one pixel of constituent pixels, and the conductive fine powder is controlled to have a particle diameter of from 10 nm to 50 μm in order to attain better charging uniformity.
Japanese Patent Application Laid-open No. 10-307457 discloses that, taking account of the characteristics of human visual sensation, conductive fine particles are controlled to have a particle diameter of about 5 μm or less, and preferably from 20 nm to 5 μm, in order to make any influence of faulty charging on images visually recognizable with difficulty.
Japanese Patent Application Laid-open No. 10-307458 also discloses that a conductive fine powder is controlled to have a particle diameter not larger than the particle diameter of a toner to thereby prevent the conductive fine powder from obstructing the development by the toner at the time of development or prevent development bias from leaking through the conductive fine powder. At the same time, it discloses a cleaning-at-development image-forming method which makes use of the direct injection charging mechanism and in which the conductive fine powder is controlled to have a particle diameter larger than 0.1 μm to thereby eliminate a difficulty that the conductive fine powder may become buried in the image-bearing member to shut out imagewise exposure light, thus producing superior image recording. It, however, is sought to make further improvement.
Japanese Patent Application Laid-open No. 10-307456 discloses a cleaning-at-development image-forming apparatus in which a conductive fine powder is externally added to toner particles so that the conductive fine powder contained in the toner may adhere to an image-bearing member in the step of development, at least at a contact zone between a flexible contact charging member and the image-bearing member, and may remain and be carried on the image-bearing member also after the step of transfer so as to stand between them, to thereby obtain good images without causing neither faulty charging, nor shut-off of imagewise exposure light. In this proposal, however, there is room for further improvement in stable performances required when the apparatus is repeatedly used over a long period of time and in performances required when toner particles having a small particle diameter are used in order to achieve a higher resolution.
External addition of conductive particles whose average particle diameter has been specified is also proposed. For example, in Japanese Patent Application Laid-open No. 9-146293, a toner is proposed in which a fine powder A with an average particle diameter of from 5 nm to 50 nm and a fine powder B with an average particle diameter of from 0.1 μm to 3 μm are used as external additives, and have been made to adhere to toner base particles with particle diameters of from 4 μm to 12 μm, more strongly than a specified extent. This practice is designed to make small the proportion of fine powder B standing liberated and those coming off the toner base particles. In Japanese Patent Application Laid-open No.11-95479, also proposed is a toner containing conductive silica particles whose particle diameter has been specified and an inorganic oxide having been made hydrophobic. This is nothing but what aims at the action attributable to the conductive silica particles by which action any electric charges accumulated in the toner in excess are leaked to the outside.
In Japanese Patent Application Laid-open No. 11-194530, a toner is further proposed which has an external-additive fine powder A with a particle diameter of from 0.6 μm to 4 μm and an inorganic fine powder B and whose particle size distribution has been specified. This practice is designed to prevent the toner from deteriorating because of any inorganic fine powder B buried in toner base particles, by virtue of the presence of the external-additive fine powder A between them. Thus, nothing is taken into account with respect to any adhesion of the external-additive fine powder A to, or liberation from, the toner base particles. In Japanese Patent Application Laid-open No. 10-83096, proposed is a toner comprising spherical resin particles in which a colorant has been enclosed and to the particle surfaces of which fine silica particles have been added. This composition is designed to endow toner particle surfaces with conductivity to enable swift movement and exchange of electric charges across the toner particles and to improve the uniformity of triboelectric charging of the toner.
Thus, with respect to developers for use in the image-forming method having the step of injection charging or in the image-forming method having the step of cleaning-at-development or in the cleanerless image-forming method, sufficient studies have not been made on external additives. With respect to proposals on developers, inclusive of those on external additives, sufficient studies have also not been made to adapt to the cleaning-at-development image-forming method or the cleanerless image-forming method.
Now, image-forming apparatus are being increasingly sought that have a higher speed and a lower cost. For example, in prevalent laser printers utilizing an electrophotographic system, personal-use first-step machines called low-end machines, which have a printing speed of 6 to 8 sheets per minute, have been made higher-speed up to a printing speed of 10 to 15 sheets per minute, and also are being made to have a low price. Translating the printing speed into the image-bearing member movement speed (process speed), the speed has been made higher from about 50 mm/sec. to nearly 100 mm/sec., and it is thought that the speed will be made much higher in the future, too.
The collection performance on transfer residual toner in the cleaning-at-development system commonly tends to decrease with an increase in the process speed. The reasons for this are that making the process speed higher makes it difficult to well control the charging of transfer residual toner in primary charging, which tends to result in non-uniform charging of the transfer residual toner sent out from primary charging and directed to the collection at development, and that it tends to become difficult to keep the triboelectric chargeability of developer from being influenced by the inclusion of the transfer residual toner collected at development. This tendency is remarkable especially in non-contact development. This is presumed to be due to the fact that, in the collection of transfer residual toner in contact development, the electrostatic force acts more effectively upon contact of the developer-carrying member with the image-bearing member and also the physical force acts because of rubbing friction, and hence any lowering of collection performance on the transfer residual toner, caused by the increase in process speed, can be compensated for with ease.
The charging performance in direct-injection charging also tends to decrease with an increase in process speed. This is presumed to be due to a decrease in the probability of contact of the image-bearing member with the contact charging member via conductive fine particles and a shortening of the charging time for which electric charges are injected to charge the image-bearing member electrostatically. Moreover, where the ratio of the charging member movement speed to the image-bearing member movement speed is maintained or made higher with an increase in process speed, a great increase in torque may cause a cost increase, and the problem of in-machine contamination tends to occur, which is caused by any scratches of the image-bearing member and the charging member and any scattering of transfer residual toner adhering to or mingling with the charging member. Accordingly, it is sought to provide a developer and an image-forming method by which any faulty pattern recovery and image stain can be prevented from occurring and any lowering of charging performance on the image-bearing member after its repeated use can be made sufficiently small, maintaining a higher process speed and keeping the charging member movement speed low.